The present invention generally relates to a current detecting method in a case of effecting the digital current control of a motor or the like by a PWM (pulse width modulation) switching system.
Since a driving control circuit of a motor using a PWM switching system employing a transistor switch is superior in power saving and is small in size, it is used in many cases in recent years. But the PWM switching system has a disadvantage in that the current waveform form pulsates by the time constant or the like of the winding, because the PWM switching system is fundamentally based on an ON - OFF control. Through higher speed of the microprocessor, the digital control which was conventionally used for the positional loop, is now considered to be of practical use for the current loop. The construction of the conventional digital current control is shown in FIG. 6.
The operation will be described hereinafter with reference to FIG. 6. In the micro-processor 11, the current value of the motor 16 is detected by the current detecting circuit 13 for each of the sampling timings generated by the sampling timing generating circuit 12 to compare the current value with the current command at this time for outputting to the PWM switching circuit 14 a PWM ratio proportional to the difference. In the PWM switching circuit 14, the voltage supplied to the motor 16 is controlled in accordance with the PWM ratio and the PWM period generated by the PWM period reference signal generating circuit 15 so as to effect the current control.
In the above described construction, however, the timing of the current sampling is asynchronous with the PWM period, with a problem that the feedback current detected is subjected to the influence of the pulsation by the PWM switching as shown in FIG. 7.